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Why would a Cessna 182T not enter a stall?
When I took delivery of a new Cessna 182T last year, I did a test flight for certification purposes. During the test flight we had to perform a power off stall but that didn't go as planned as it was simply impossible to stall. What happened is this: when the airspeed dropped well below the power off stall speed we simply started to sink slowly with a nose-high attitude at about 35 KIAS. This "mushing" went on for what seemed ages before I eventually applied power and pushed the nose down to gain airspeed again. We tried it again after that and the same thing happened. I had an instructor with me who has thousands of hours in a C172 and he had never experienced something like this before.
The odd thing is that this only happened during that flight. During later flights this didn't occur anymore.
There was a passenger in the back seat, fuel tanks only half full so the CG was more aft than usual, but well within limits
Ever since that flight I've wondered:

What could cause this to happen? (My guess is it is CG related)

And most importantly:

If I would have continued this "mushing" flight, would it be possible to have entered a flat spin or a simple "drop out of the sky" like the infamous Air France 447?

Did you happen to notice your trim setting?  It is possible that an extreme down-trim setting could make it harder to stall the plane.   Given that your CG may have been slightly aft, you may well have had more nose-down trim than usual.

That's a very good point. I didn't consider this to be a factor.

I flew a Rallye 100 & 220 for many years which had slats, and stalling them (as opposed to the mushing you describe) required an almost aggressive entry.

I think you are confusing a departure (OCF) with a stall.  You are out of control the moment that normal control inputs result in unexpected results.  IE, when a wing rolls off, or the nose suddenly drops during a stall.  However, many aircraft are capable of remaining controllable while exceeding their critical AoA in a stall; therefore, you cannot determine a stalled condition based solely on the responsiveness of the controls.  Mushing may very well be a stalled condition, and I'd argue that if you pulled the stick back and the nose didn't accordingly track up that you were technically OCF.

@abelenky -- re "It is possible that an extreme down-trim setting could make it harder to stall the plane." -- it's actually the opposite, right?  Assuming the plane has a conventional trim tab rather than a moving horizontal tail-- if I'm wrong about that, then never mind.  Anyway, in the case of the trim tab, then nose-down trim raises the back of the trim tab.  With yoke full aft, the hands-off position of the yoke is irrelevant, as is the felt force on the yoke, but the little tab sticking up into the airflow gives you a little more nose-up pitch authority.

(Yeah I know it's a 10-year-old topic but-- ;)

Link to flight manual of 185T-- see page 7-9 -- http://rainierflightservice.com/aircraftdocs/182-poh.pdf -- it's a trim tab, not a moving horizontal tail

I think this is more common than you might think in a C182. The 182 is much more nose-heavy than a 172 (which is particularly noticeable in the flare) and this seems to limit the amount of upward pitch authority the elevator has at low speeds. The only stalls I've done in a 182 are much as you described - you can hold full back-elevator and you just sort of sit there sinking.
Spinning could be an issue if it becomes uncoordinated, but a 182 is a very stable airplane and unlikely to enter a flat spin unless the CG is beyond the aft limit or severely uncoordinated. Perhaps someone else can weigh in further on that possibility, though.

I have the same experience in the Diamond DA20-A1, it's virtually impossible to stall by reducing power and maintaining altitude (keeping the stick full aft, it'll buffet and start to descend, but not enter a full stall), and will require quite the manhandling to enter a spin.

Same experience in the C182; it's also much the same story in the AG-5B Tiger and DA-40. All three have pretty favorable stall characteristics, and a mid/forward CG in the 182 does still leave it pretty nose-heavy.

I wouldn't call this behavior favorable. A nose drop when a stall occurs will break the stall by itself and would be more obvious (and beneficial) to the pilot than mere sinking in a nose-high attitude. At low altitude the pilot could very well be unaware of the stalled condition and wouldn't see the ground approaching because of the attitude.

Piper PA28 (and to lesser extent the PA32) exhibit similar behaviour.  They just kind of descend while fluttering slightly.  Not at all like the HARD break the C172 I trained in did.

@PhilippeLeybaert By favorable, I think egid was mainly referring to how stable and controllable the plane is in a stall vs. wanting to go into a spin. The Cirrus SR20 G2 I did my private in always dropped a wing, which was decidedly unfavorable. Whether or not a break is preferable, I guess is a matter of opinion.

Less upward pitch authority should make it more likely to drop nose, not less, no?

Some aircraft don't have enough elevator authority to enter a real stall if the have a nose heavy CG. The wing fails to achieve the required angle of attack, and the plane mushes.
The solution for these kids of planes is not to enter the stall from MCA, as is typically taught. Rather, enter from straight and level flight at Vx, and break into a full stall by sharply moving the yoke to its full aft position.  This should result in a significant nose-up attitude and exceed the critical angle of attack, and you will see and feel a sharp break. Remain coordinated throughout the maneuver, and recover by releasing back pressure, and don't let the nose get too low.
This method can also be adopted for aircraft that don't have enough  elevator authority to do a turning stall.
If you're doing this in a power plane (rather than a glider), you'll probably wind up adding power in the recovery.

A stall occurs based on angle of attack, not speed. If the plane tends to nose over near stall speed, the angle of attack may not increase enough for a full stall, even though the plane is losing altitude. If you want to perform a low speed stall in the plane, keep pulling the stick back to maintain altitude or climb slightly as the power is eased off. Use the rudder to keep the wings level to reduce the likelihood of a spin.

That's exactly what I did. Maintaining altitude by pulling back more and more until there was no more pitch change (yoke full back). In a C172 the nose would've dropped but the C182 just started sinking, nose-high with yoke full back.

Interesting! I guess you could throw some weight in back (within CG limits, of course), or use a little power.

Doesn't matter how far back you've got the yoke. As xpda said, it's the angle of attack that controls a stall. In this aircraft, you ran out of elevator control before you ran out of angle of attack. You can make it stall by entering the stall a bit quicker, while you still have more ability for your elevator to increase the angle of attack. The C172 would do this as well.
If you want to make the 182 break into a stall, do that again, but while you're fluttering, goose the power just a little bit to allow the elevator to increase the AoA. It'll stall.

@Shawn: If you run out of elevator authority, the nose pitches down and the aircraft starts to descend. Which is (unless I am misunderstanding the description) contrary to the question which says “started to sink slowly with a nose-high attitude”, i.e. elevator needed enough authority to keep the nose up.

please don't use the rudder to keep the wings level. use the rudder to stay coordinated

There are some very good answers here, I enjoyed reading this post.
I would just like to add in a lot of aircraft, if you are aggressive on the rudder (in order to keep in balance at all costs, i.e. every time a wing moves slightly down you step on the sky), it is very difficult to enter a spin. The opposite is also true, if you are flying along in mushing flight and give the rudder a good kick a spin is often the result.
Different aircraft will handle differently in this mode, in some like the 182 it will be relatively easy to stay there, others could end up in a spin if the smallest of mistakes were made.
Alan Bramson used to call the aeroplane a "tin parachute" when in this mode and I believe in the olden days it was used to descend rapidly through holes in clouds (!) without building up too much speed. Although its not really a parachute because if you hit the ground in this mode the sink rate would usually be so high (in excess of 500 feet per minute), the ground would obliterate you.
To fully answer the other half of this question I think its most probably CG related and that being a new aircraft its easier than normal to keep in balance being very clean etc.

I assume that 500 feet per minute is a typo? The landing gear on most aircraft can survive a 1000 feet per minute drop on the runway :-)

I'm not sure actually. It was a figure I plucked out of my head. 500 feet per minute is 8.3 feet per second. I wouldn't like to try it - that is get a proper 500 feet per minute descent going and fly into the ground even if nose up. The BRS parachute system fitted to the Cirrus descends between 680 and 1500 feet per minute and the aircraft is usually wrecked. So the next time I'm flying at night and the engine fails I don't think I would go into tin parachute mode even if my descent rate was 500 feet per minute ( I would think in most types it would be a lot more than that). Regards Phil

Airliners need to be certified to survive a 600 fpm drop on the runway without any damage. They can usually withstand more than that. But as you said, going down in "tin parachute" mode will be in the order of a few thousand feet per minute.

My last flight involved some stall training in a PiperSport LSA, and it actually did take considerable conscious input to make the plane move past that "mushy" stage and depart completely (it would nose down quite handily when you did though).
I concur with other answers; there is a difference between a "stall" (exceeding critical angle of attack and thus losing lift and/or full attitude control) and a "departure" (losing most or all attitude control). If pulling back on the stick or yoke won't continue to pitch the nose up, you've stalled, but you haven't departed completely until you can't push the nose back down (or it noses down even with full nose-up elevator applied).
Many small planes with "normal" or "utility" category ratings (or "limited" light-sports) are designed with a large amount of "departure resistance"; their aerodynamic design and mass distribution makes it hard to fully depart the aircraft and enter a nose-down stall (or a spin, spiral dive etc). This also makes intentional maneuvering near the edges of the flight envelope more difficult, as the plane will want to settle back into a stable configuration rather than let the pilot potentially push further into departure.


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Do helicopters have an auto hover system?
I'm not a helicopter pilot. The way I understand it, helicopter pilots have to constantly make small adjustments to make a helicopter hover steadily at a certain point in space, just like fixed wing pilots flying in close formations.
Are there any systems which, when engaged, will measure the helicopter's movement with gyroscopes and accelerometers, and automatically compensate for small deviations?

Some helicopters like the CH-65 have hover augmentation systems. The Bell 412 has an automatic hover button. I'll see if I can find some information for a more complete answer, but a heli-pilot may be able to give a better answer than I can.

Related: Do rotorcrafts like helicopters have autopilots?

Also: http://aviation.stackexchange.com/questions/23977/why-not-simplified-fly-by-wire-helicopter-controls

Yes, advanced flight directors and autopilots for helicopters can control the aircraft in all axes, and often they have modes for hovering built in, including maintain altitude, lateral veloctiy hold, hover, and/or autolevel.
Hovering is difficult because it requires more complex controls than maintaining altitude in an airplane.  Helicopters have a different set of control axes than fixed-wing aircraft.  Fixed wing aircraft are controlled in pitch, roll, and yaw, with thrust as an extra variable.  On a helicopter the flight path is controlled through cyclic (roll and pitch), collective, and throttle, with the anti-torque pedals as well to control yaw.  Hovering the aircraft requires controlling of the cyclic to keep the aircraft over its target with no lateral movement, and collective and throttle to maintain constant altitude, although the exact procedures are a little more complex.
While not present on low-end helicopters due to a combination of complexity and lack of necessity, advanced autopilots for helicopters do exist.  Like on other aircraft, there is a wide variety in autopilots between simple functions to maintain attitude (often called one or two-cue guidance depending on the number of axes used) to complex FMS-integrated three-axis modes (which would be "three-cue" guidance). The FAA Helicopter Flying Handbook states, for example, that  "The most advanced autopilots can fly an instrument approach to a hover without
any additional pilot input once the initial functions have
been selected."
On the Agusta AW139 helicopter that I'm familiar with (although I'm not a pilot), they have several modes that provide this functionality:

an altitude mode to maintain a given altitude
an altitude acquire mode to smoothly reach a desired altitude
an autolevel mode to level off at the end of an approach segment
a lateral velocity hold mode to maintain slow lateral motion or no lateral motion
more exotic search and rescue modes providing capabilites to transition to a hover, mark-on-target descents to predefined altitutdes, etc.

A traditional hover would be achieved by combining a low lateral velocity mode with a zero vertical speed mode like altitude or autolevel.

One other reason that hover is so difficult is - most aircraft in flight have a natural stability (excepting deliberately unstable fighter aircraft) from aerodynamic pressures on the aircraft in flight. A helicopter in hover has no natural stability - it is unstable in all three axes. Somewhat like trying to stand up on a sheet of ice. Also the reason that helicopter pilots tend to get in forward flight as soon as possible - it provides some stabilization.

Yes.  Some helicopters, especially rescue, have auto-hover.
Here are some examples:

More to follow...
I tried to find some videos for you, especially of the Bell 412 auto hovering on YouTube which seems to have been removed.
The AW-101, Bell 412, Bell 525, S92 and others have this capability.

the video is not available anymore :(

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How is the static pressure in the pitot static system reconciled when there are two outside static pressure ports?
On airplanes where there are two static pressure sources (ports) on each side of the fuselage what is the result of the static pressure as compared to aircraft with a single port. (I'm not talking about the alternate static port which is usually inside away from freezing weather).
For example, from the diagrams I've looked at it appears that the dual static ports are connected to a common line such that the pressures from each port are averaged out, such that if one port has 14 psi and the other 10 psi the result will be 12 psi. Is this how it works?
Another example to help illustrate my question, imagine an aircraft with two static ports on each side exactly symmetrically opposed is in a forward slip attitude, (ignore ram air pressure for this example), does the static pressure from each side when mixed together cancel out the error caused by the slip attitude as compared to an aircraft with a single static port in the same attitude? Does dual symmetrical static ports correct error of a slip to give you the actual static pressure as if the aircraft was flying perfectly coordinated? (Again, this question is just about the static pressure, not ram air).
If anyone has a more concise way to ask this question please chime in.

With multiple ports connected to a common line the pressures will not average out, but a flow will be induced that will accelerate until the static pressure matches the lower pressure and the total pressure matches the higher one. Then depending on geometry of the tube the instrument might use the lower value (simple T junction and constant thickness) or something different (but always less than the higher pressure).

"such that if one port has 14 psi and the other 10 psi the result will be 12 psi. Is this how it works" That is kind of how it works, but without doing a lot of fancy math all you can really say is that the sensed static pressure will be somewhere between 10 and 14 PSI (which by the way would be a HUGE difference, really you are only looking at a difference of maybe a few tenths/hundreths of a PSI).

@RonBeyer yea, I just pulled those numbers out of thin air for simplicity. So while dual static ports may not completely cancel out the error caused by the slip, it is more accurate than an aircraft with a single port in the same uncoordinated attitude?

@Devil07 The accuracy is probably negligible in most cases, especially during straight and level flight. However, two ports offers a valuable redundancy in case of obstruction (ice, FOD, etc) that having only a single port simply can’t give you.

On airliners they are separate measurements - one feed the captain side and the other feed the copilot side. On GA I'm not sure.

@kevin - On airliners (at least on Boeing) the 2 (sometimes 3 or 4) static parts are not independent, but split between the two pilots. The most common failure is icing and when this occurs each pilot looses a separate set of instruments. This is designed to allow pilots to quickly realize a contradiction of their instruments and react to the situation sooner. Part of the recurrent training is to identify bad ports from the discrepancies.

I work on corporate jet aircraft and perform troubleshooting, testing, and certification on pitot-static systems.
The reason aircraft have static ports for each pitot static system is to equalize or average out the small differences in static pressure caused by crosswinds when the aircraft is in flight. The difference in pressure caused by crosswind components on the static systems is not in the psi range, rather it is in the millibar or 1/10 of inches of mercury (inHg) range.
14CFR, Part 43, Appendix E, Table IV "Pressure-Altitude Difference" lists several baro-scale test points and associated altimeter readings used when certifying the altimeters. From the chart you can get an idea of how a difference of 1/10 in-HG  will case a change of 90 to 100 ft in the altimeter reading (you can Google this table yourself).
An aircraft could easily have a difference in barometric pressure between the up-wind and down-wind sides of the aircraft when flying in a crosswind and this would cause an error of 100 ft or more on the altimeter reading if there was only a single static port located on one side of the aircraft.
On large aircraft which require 2 pilots, there are two completely separate pitot and static systems for each of the flight crew. Each pilot's instrument panel has its own altimeter and airspeed indicator (or air data computer) and display of altitude and airspeed information. There is a single pitot tube for each crewmember's instrumentation located on his side of the aircraft, but each crewmember's static system has two ports, one  on each side of the fuselage which reduces the error effects of crosswind on the altimeter (static) systems (total of 4 static ports).
For peace of mind and safety, the flight crews continually compare (cross-check) each other's altimeter readings and if they see an error of 80 ft or more between the two systems they have an uneasy uncertainty about which altitude is correct and they will bring it to the mechanic's attention.
A blocked static port is a funny thing to troubleshoot, because a flight crew will say they saw a big altitude difference on one leg of the trip, then the next couple legs it was fine, then it had a split again. Next crew will fly and say there were no problems, followed by another crew complaining about splits. If you question the crews and ask if they were experiencing heavy crosswinds when they had the split they will say, "hmm, now that you mention it... yeah, we had some heavy crosswinds".
We then connect our pitot-static tester to each side's pitot tube and one of each side's static port, block off the opposite static port, and bring the airspeed and altitude up on the tester. What we are looking for when we connect the tester to the obstructed static port is a split between the captain's and first officer's altitudes. (Actually the obstructed side will start to wind the Airspeed indicator up really fast and can damage it if you don't stop the test quick enough).
One side is obstructed and the difference in static pressure caused by the crosswind is throwing off the altitude reading on one of the altimeters.
Two ports per system is to reduce errors in the altitude indication.

@JerryRig I read the whole answer, thank you. Great info.

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If we are cleared for an ILS approach, can we descend according to the approach chart？
For example VCBI，ILS Z 04. We have ATC clearance:

Descend and maintain 2300 feet, Cleared ILS RWY 04 approach.

Can we descend as profile 1700 and then 1500, and intercept glide path? Or maintain 2300 and intercept glide path? Or even maintain present altitude - for example 2700ft - and descend on glide path?

Descend and maintain 2300 feet, Cleared ILS RWY 04 approach.

This clearance allows you to descend to 2300 FT MSL right away.  When you are on a published part of the procedure (i.e. on the localizer with positive course guidance) you can then descend to the altitude shown on the chart for that segment.
You can choose to remain higher to capture the glideslope or descend to the MEAs when allowed.
I am a firm believer in a stabilized approach and staying higher to capture the glideslope helps to accomplish that.  There is an issue, however, with capturing the glideslope early and following it down and it has to deal with warm weather.  In warmer than standard weather, it is possible to be on the glideslope and find yourself below the MEA for that segment of flight.
In that case, it may be best to descend to the altitudes listed on the chart until you are at final approach fix altitude and then capture the glideslope and descend.
In your specific example, I Would descend to 2300 FT MSL and capture the localizer.  I would stay at 2300 FT and descend on the glideslope.  In really warm weather, I would be cautious of my altitude crossing LAROD and GOPRU

in cold weather I keep higher altitude to capture the glide slope.When I am in glide slope cold weather is nothing to me.

I agree with you @Bianfable

Regarding, "This clearance allows you to descend to 2300 FT MSL right away..."  That clearance also requires you to descend to 2300', perhaps to vacate an altitude that the controller needs for somebody else.  Once established on the localizer, the published altitudes are generally (not always) "at or above" altitudes, so in those cases you can remain at 2300' to capture the GS there, or descend to whatever is on the profile, and either way you're meeting your clearance.

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Does the cross country for instrument rating have leg distance requirements?
I already have my commercial rating so I don’t need to do that long XC.
IR-Is this 250nm requirement for the total distance of the flight?

Sec. 61.65
Instrument rating requirements.
(d) Aeronautical experience for the instrument-airplane rating. A
person who applies for an instrument-airplane rating must have logged:

(ii) Instrument flight training on cross country flight procedures,
including one cross country flight in an airplane with an authorized
instructor, that is performed under instrument flight rules, when a
flight plan has been filed with an air traffic control facility, and
that involves--

(A) A flight of 250 nautical miles along airways or by directed routing from an air traffic control facility;
(B) An instrument approach at each airport; and
(C) Three different kinds of approaches with the use of navigation
systems.

The 250NM is for the total distance of flight. However it also needs to be long enough to fit in the definition of a cross country flight for flight training purposes, in other words you are going to need to fly more than 50NM from your home airport.

(ii) For the purpose of meeting the aeronautical experience
requirements (except for a rotorcraft category rating), for a private
pilot certificate (except for a powered parachute category rating), a
commercial pilot certificate, or an instrument rating, or for the
purpose of exercising recreational pilot privileges (except in a
rotorcraft) under § 61.101 (c), time acquired during a flight -
(B) That includes a point of landing that was at least a straight-line distance of more than 50 nautical miles from the original point of departure


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Knots vs MPH, is there some standard for when to use which unit?
In looking at all the sources of aviation data, including POH's, instrument dials, instructional materials, FAA publications, METAR's, flying magazine stories, YouTube videos, etc., I see no standard usage of either knots (nautical miles per hour), or mph (statute miles per hour).  It seems many people use them not interchangeably, but mix them up seemingly randomly.
Is there any kind of agreed upon standard about when to use which system of units?  It's easy enough to convert one to the other (well actually a lot easier to convert knots to mph), but it seems this is just a latent trap for mistakes in navigating and, more importantly, in airspeed errors.

Knph is what you see on aircraft instruments and hear over ATC. Mph is probably used just so that a common person would know what is happening.

I agree with @SMSvonderTann, all the instruments, FAA publications, METAR as well as the ceritifications criteria (e.g.: stall speed > 61 kts for CS23) etc are given in knots, not in status miles per hour!
Gliders and russian airplane though use the metric sstem, Km/h for speed and m/s for rate of climb.

My airplane's ASI reads in mph.

There is no rational reason to use statute miles, ever.

Be careful with absolute statements like "all" instruments.  The 1963 Cessna I fly displays MPH.  I don't like it.  I still call out airspeed saying "knots".  I know it is "not" true, but I won't try to break my habit at this point.  I know what i am doing, and I am targeting the correct number, it's just that "miles per hour" is too much of a mouthful to say...

Also, remember that the term knots is defined as nautical miles per hour. Knph would be knots per hour which would be nautical miles per hour per hour which is a definition for acceleration.

@MichaelHall - I fly school, club, and rental aircraft of various ages. During my equipment checks and run ups, I’ve gotten in the habit of verbally calling out what measurement units my a airspeed indicator is in. I also make a Takeoff And Landing Data (TOLD) card to remind me of my Vspeeds. It helps to keep things straight between the different aircraft. Especially since some are in knots and some are in mph.

@GBH -- re "Gliders and russian airplane though use the metric sstem, Km/h for speed and m/s for rate of climb." -- not any of the twenty to thirty gliders that I've personally flown in the USA!   Even those of german manufacture.  Most had the ASI in mph, some of the newer ones had the ASI in knots, and almost all had the variometer (VSI) in either feet per minute or knots, which are almost the same thing.

According to the FAA Flight Navigators Handbook (the Flight Navigator Cert is rarely issued any more to my understanding but none the less).

The standard unit of distance for navigation is the nautical mile
  (NM). The NM can be defined as either 6,076 feet or 1 minute of
  latitude

It goes on to talk about speed

Closely related to the concept of distance is speed, which determines
  the rate of change of position. Speed is usually expressed in miles
  per hour (mph), this being either SM per hour or NM per hour. If the
  measure of distance is NM, it is customary to speak of speed in terms
  of knots. Thus, a speed of 200 knots and a speed of 200 NM per hour
  are the same thing. It is incorrect to say 200 knots per hour unless
  referring to acceleration.

It seems it comes down to what you measure distance in, then simply report the appropriate speed units.
Most pilots will calculate (and talk in units) based on the instrumentation in their plane. Some airspeed indicators contain both knots and MPH however its not uncommon for them to contain only a single unit (generally older GA planes is where I have seen MPH instruments). POH's will generally contain what ever unit the air speed indicators in the original build of the plane contained (some POHs will contain both units). Just to show some examples, Mooney reported speeds in MPH in their older POH's while say a Piper from that era shows Kts in its POH. For example this indicator contains both units.

(source)
In the end of the day its most important that the units are not mixed up and or used interchangeably.
As for the errors in airspeeds this is one of the reasons that air speed indicators have color queues to indicate things like stall range, flap extend range and smooth air operating range. This way if a pilot does accidentally confuse units there are other things to rely on. (Side note there are some important speeds that do NOT appear on airspeed indicators that should always be noted).
-Edit-
To answer the comments below.
I report distance in NM since thats what foreflight uses on its ruler function as well as its ring function (at least the way I have it set up). Most plotters I have seen are in NM so even if I had a paper map I would be reporting NM as well.
Im not sure the FAA has official declared that Knots are the only acceptable unit. MPH airspeed indicators are still considered air worthy so they must at least accept it on that level. There have been pushes to standardize instrumentation after the issues caused by the eastern vs. western attitude indicators but with the advent of glass cockpits most of these things can now be configurable in software and changed by a simple setting. The FAR's (such as 91.117) report speed in both KTS and MPH and that is the letter of the law from the FAA.

Not that it's a huge difference, but when pilots report positions in flight, do they really mean NM as opposed to statute miles?  So a 2 mile final is ~12,000 ft and not 10,500 ft?

That's interesting about the Mooney.  I wonder if the convention has slowly morphed over the years or was it by some decree of the FAA?

Useless trivia: The Mooney 201 was named the "201" because it could achieve 201mph (not knots) in level flight which was (and still is) a notable accomplishment given it only has a 200hp engine. You can see why Mooney favored mph...a "Mooney 175" doesn't quite have the same ring to it and Cessna already had a 175. The Mooney 252 has a similar story and was certified to 28,000' just to achieve marketing's target number: 252.  Legend has it that the 231 was called the 231 for the same reason.

Knots are used almost universally for marine and aviation navigation because a nautical mile relates directly to mapping of the earth.  A nautical mile is equal to 1 minute of latitude making it very easy to use Knots for speed and nautical miles for distance when already the earth's mapping uses degrees and minutes of latitude.  Each degree of latitude is equal to 60 minutes of latitude.

Knots, or nautical miles per hour have long been the standard units of velocity for both nautical and aeronautical navigation.  Despite the SI adoption of the meter as a standard unit of length, nautical miles have been very popular with cartographers and thus made their way into use for navigation of ships and planes.
A nautical mile is defined as one minute of lattitude.  Therefore the circumference of the planet is 360 X 60 = 21,600 nautical miles. So a nautical mile is 24,856 / 21,600 = 1.1516 statute miles or 2025 yards or 6076 feet.

According to the FAA, a NM is 6076.1 ft.

That's going to depend on what value you use for th circumference of the Earth in statute miles.  I'm using 24,874 sm here, which is the Equatorial circumfrence.  The FAA is using 24,856 sm, which is the Meridional (pole to pole) circumference.   I argue my answer would be more accurate.

I'm pretty sure the nautical mile is defined as exactly 1852 meters (about 6076.1 feet).

@CarloFelicione you can't use the equatorial circumference. "A nautical mile is defined as one minute of lattitude" Latitude = a circle over the poles. So that's the circumference you need to use. The equatorial circumference is related to longitude.

As other answers have noted, the nautical mile is equal to one minute of latitude. This has an advantage when navigating using a chart that spans a large latitude range.
Such a chart cannot have a constant scale across the entire latitude range of the chart. But it will typically have longitude and latitude lines marked, and you can use a ruler to read off the length on the chart of a degree of latitude close to where you will be flying. This is obviously more significant when navigating over a long distance.
Given this convenience of using nautical miles as a unit of length, the knot becomes a convenient unit of speed.

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Is it possible to record the screen of Garmin avionics?
Is there built-in functionality to record the screens of Garmin avionics?  G1000, G3X, etc.  I'm trying to play back video recordings of flights and want to see what the avionics are doing while I fly.

According to this forum post on Pilots Of America (from 2010) a G1000 has the ability to log the data. Then you can upload it to cirrusreports.com, get mapping, parameter graphs and even replay the flight.
This is 14 years old and things may have changed since then, but I doubt that Garmin has removed a feature (you never know...) though you may have to find a different way to view/play back the data.
Note: no affiliation whatsoever with any of the sites mentioned, it was literally the first result of a DuckDuckGo search for "screen record Garmin G1000".


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Why are delta wings not commonly used in commercial airliners?
Delta wings are most commonly seen on military aircraft. I understand that delta wings are ideal for supersonic flight, and provide high maneuverability.
Having high maneuverability and speed are good characteristics for commercial airliners. But, most of the commercial airliners do not use delta wings (Except for Concorde and Tu-144).

Are there any specific reasons for this?
Are there any regulations that prevent using delta wings in commercial airliners?
Does it cost more in maintenance?

delta wings aren't really optimal for subsonic performance, and most commercial jets don't need amazing manoeuvrability

Having High Maneuverability is not a good characteristic for a commercial airliner, as maneuverability and stability are competing interests.   Airliners want stability, and do not need maneuverability as they generally don't bank more than a few degrees, and make large, gentle, well-planned turns.

Similarly, speed is not an important characteristic for a commercial airliner.   The main focus is on efficiency.  Airliners today actually fly slower than they did a decade or more ago.  But they are more fuel efficient, and therefore cost less to operate and more profitable.

@ Shortstheory & @ abelenky thanks for the information

@abelenky: speed is good (to an extent). Aircraft have an optimal cruise speed, and cruising below that is inefficient.

(Originally posted (incorrectly) as an answer): Some historical trivia: an airliner conversion of the Avro Vulcan, with a larger fuselage, was considered in the early 1950s. I have seen it referred to as the Avro 700 or 722 Atlantic.
And the video of Roly Falk barrel-rolling a Vulcan at the 1955 Farnborough air show is worth a look: http://www.youtube.com/watch?v=GPuTgcrA2Zs

There's a thread on airliners.net about the same topic.

One of the most prominent and useful aspects of the delta wing is the fact that it increases an airplane's critical Mach number (the speed at which some air passing over any part of an aircraft - usually the wing - reaches Mach 1.0).

By increasing the critical Mach number, it makes supersonic flight much more feasible on delta wings than on conventional wings.
Delta wings are particularly happy even at a high AoA (angle of attack). This is because of the reaction lift generated by the wing, and the vortex created at the leading edge of the wing. Hence, airplanes with delta wings have a much higher tolerance to stalling. This makes it easier to attain higher vertical speed, also known as "rate of climb" safely.
But on the flip side:

One of the downsides of deltas is that they create more drag, because of the bigger surface area, but they do have a bigger internal volume (for fuel, for example) which could lead to a smaller fuselage, or more space in the fuselage for other things. Another disadvantage is higher landing and take off speeds.

The last point is particularly important. The Concorde's already small number of usable airports was reduced because the runways needed to be long enough to accommodate for the 160 KIAS landing speeds (for comparison, typical turbofan-based airliners lands at 130-140 KIAS).
As a result of the higher takeoff/landing speeds, the  cost of maintenance of vital components such as the brakes and tyres is higher. Moreover, the high landing speeds can even lead to greater stress on the airframe. So, indirectly delta wings do contribute to a higher cost of maintenance.
Other disadvantages of delta wings I can think of is the lack of a flaps/slats system. Delta wings don't require flaps as they can simply reduce their airspeed by increasing the AoA, but the lack of flaps reduces their subsonic efficiency, and increases the takeoff distance and takeoff velocity. For instance, the Concorde's delta wings produced very little lift during takeoff prior to rotation, necessitating high takeoff speeds of nearly 220 knots!
As an answer to your second question, I could not find anything about regulations relating to the use of delta wings on airliners.
All in all, delta wings work well for niche purposes such as supersonic aircraft, but their disadvantages and poor operating economics make them impractical for subsonic flight.
However, as you can see here, the now-scrapped Boeing Sonic Cruiser attempted to make the best of both worlds by combining a conventional wing with a delta wing, to form a wide, "cranked-delta" wing design.

Several good points, but your statement that a high angle of attack allows a higher climb speed is wrong. A higher climb speed is achieved with more engine power.

Hmm, thanks for the comment. I meant to say that delta wings are less prone to stalling than conventional wings, and hence high AoA's are possible; I'll make the required edit to make it more clear.

Well, not quite. Yes, they will stall at a higher AoA, but stall they will. And if they have a straight leading edge (as opposed to a cranked one with less sweep outboard), they will produce a strong pitch-up moment around and beyond stall, making recovery very hard.

@shortstheory What's stopping designers from giving delta wings flaps and slats? IIRC modern delta-wing fighters (I think especially the fly-by-wire ones) like the Mirage 2000 and newer models have slats and flaperons. My knowledge is very limited, but remember reading somewhere that a lifting canard helps reduce the landing and takeoff speeds of delta-wing designs.

@markvgti, I'm sorry I didn't clarify a bit more. Indeed, many delta-wing based airplanes have flaps and slats, but the original Concorde did not (IIRC the never-produced Concorde B model did have a flaps/slats system). Most airplanes with delta wings don't need flaps/slats, so I suppose engineers don't install it because of weight issues.

And, yes, what you read about the canards is completely right. I suggest you check out the XB-70 Valkyrie and the Tupelov Tu-144.

I imagine flaps and slats on a delta are not seen as much, because there isnt as much room to play with the moment distribution around the wing. With a slender wing a shifting pitching moment is relatively unimportant and can be compensated; in a delta wing managing the pitching moment is already a critical part of maintaining pitch stability, and you cant just go wild with it as you would by extending a massive flap.

Generally, having as little sweep as possible is the best way to achieve high lift at low drag. Also, regardless of flight speed, a subsonic leading edge gives the best lift/drag ratio. If the sweep of the leading edge is higher than the angle of the Mach cone in supersonic flow, the flow around the leading edge is still like in subsonic flow, and this creates suction, thereby reducing pressure drag. If the cruise speed is Mach 2, the sweep must be bigger than 60 degrees, and with that sweep the lift at slow speed would suffer. Lift goes down with the cosine of the sweep angle at the same angle of attack.
Also, at high angle of attack the high sweep angle produces a vortex on the upper surface which reduces pressure (= increases lift) and prevents flow separation (= allows for higher angle of attack). Having more chord inboard helps to utilize this lift better and gives you more space for the wing spar and fuel, even with the low relative thickness that supersonic flow requires.

A subsonic airliner wing needs only moderate sweep to reduce Mach effects even with relatively thick airfoils (14% t/c versus 4% to 6% for supersonic airfoils), which gives you a lot of internal volume for fuel, the wing spar and high-lift devices. Going to a delta wing would help with gust sensitivity in low-level flight (that is why you see delta wings on subsonic aircraft at all), but it would increase the surface area and, consequently, friction drag.

FAR Part 25 would be the relevant regulation, and it (thankfully) leaves it to the applicant how conformance is achieved. The reason why you see no delta wing on subsonic airliners is purely technical.

If you take all together, the moderately swept-back cantilever wing with high-lift devices is the best way to fly Mach 0.8, and the delta wing is the best for flight at Mach 2.

swept-back cantilever wing with high-lift devices is the best way to fly Mach 0.8 uhm, you mean that the devices are there, not that are used to fly at 0.8M, right?

@Federico: Yes, you only need high-lift devices at low speed, but need to carry them around for the full flight. And storing them away is non-trivial. Having a relatively thick wing helps immensely with that. The delta wing has the high-lift device built-in (high leading edge sweep + high inboard chord). A swing-wing combines both, but adds a lot of weight, reducing payload.

"Going to a delta wing would help with gust sensitivity in low-level flight" But what about microbursts/downdrafts? Surely those broad, great surface area wings would "absorb" more downwind and thus be pushed downwards? This would be bad anywhere, let alone in low-level flight.

@DrZ214: Wings don't "absorb" wind. The delta wing is less affected by the angle of attack change caused by a vertical wind speed change than a straight wing. But the real danger in microbursts at low level is the change in horizontal speed, and here both wing types will react in the same way.

The primary design driver of a commercial airliner wing is efficiency, and second to that is speed. While delta wings are good at high speeds, they are not very efficient. Also, while an airliner should certainly be maneuverable, they do not need to "do a barrel roll" or make 9G turns light a fighter jet.
It was mentioned that delta wings have a large surface area. This helps to distribute the weight of the aircraft more, which leads to a low wing loading. This is one factor in the higher maneuverability of the delta wing. Compare the wing loading of the F-15 at 73.1 lb/ft2 with the wing loading of a 737-800 at 98.7 lb/ft2. The delta wing allows the F-15 to have lower wing loading and thus much more maneuverability. The design also keeps this large wing area to a low wingspan, which means that high-G maneuvers cause lower stress on the wing. Airliners have plenty of maneuverability with their current configuration.
This large area and high speed performance is at the expense of efficiency. The lift to drag ratio (L/D) of an F-15 is around 10, while commercial jets get over 16 (see this table for some examples). This shows the huge advantage that a non-delta wing has at subsonic speeds. Modern aerodynamics have allowed commercial airliners to reach high subsonic speeds (Mach 0.8-0.9) while remaining very efficient. While fighter jets use aerial refueling to compensate for burning more fuel, this would be a bit unrealistic for a commercial aircraft.
Low speed performance is also a factor. Both wings are able to use flaps and slats, but landing speeds for fighter jets are generally higher than commercial aircraft. Being better suited for high-speed flight, delta wings are less effective at low speeds.
As far as maintenance is concerned, there are fewer differences. Since the wingspan would be lower, this would put less load on the wings. Reducing the load could be turned into a maintenance advantage, but there are many more factors in determining maintenance cost.

Tex Johnson disagrees that airliners don't need to do barrel rolls.

@cpast Well his point is that you can do it, and it is impressive. But you can sell airplanes without it, and the people in charge don't like it (for some reason). Also, that picture is one of my desktop images. : )


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What's the maximum size of cargo that fits into the Chinese Y-20 cargo plane?
Somewhat related to this Skeptics question, what's the maximum size object that fits through the door into a Y-20 cargo plane?
Would an 18 x 4 meter cylindrical item fit in? I can find the maximum weight an Y-20 can carry, but that's not terribly conclusive with respect to dimensional limitations of its cargo hold.
The Y-20 can carry Type 15 tanks, but those are only 3.3 metres wide (and 2.5 metres high).

First of all this picture from Reddit gives a general idea of the size of the Y-20 compared to C-17:

(link to picture source) While the Y-20 is in the foreground, the C-17 still looks about the same size, so the Y-20 would be smaller than the C-17.
This article: globalsecurity.org - Y-20 Kunpeng - Design provides a comparison of cross sections of the C-17 and Y-20, which does coincide with the percieved size difference seen in the picture above:

The article further notes:

Compared with the Il-76, the Y-20 seemed to have a shorter wingspan and a shorter, but slightly wider, fuselage [5.4 meters versus 4.8 meters]. The Y-20 is larger than the Airbus A400M and has about the same fuselage diameter, but is much smaller than the Boeing C-17.

and:

The IL-76 has a narrow cargo compartment that limits the size of the load. The Xian Y-20 is closer to the of C-17 cabins, i.e. within 4.0-4.5 meters height and width and approximately 20-25 meters in length.

The cargo hold of C-17 is 548cm wide, and if the picture above is reliable, the given appriximate dimensions of the cargo hold of the Y-20 match (well this is pretty much a circular argument).
If the information above is correct, even with some play, Y-20 should be able to fit a 18x4 meter cylinder (as long as the 18 is the length, and 4 is the diameter). The rigidity and other properties of that object might however make it impossible to load it on the Y-20, as it might require substantial reinforcements around it.


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Are there any aircraft with a 4-wheel nose landing gear and a 16-wheel main landing gear?
Are there any aircraft with a 4-wheel nose landing gear and a 16-wheel main landing gear? I need this to answer my review of an aircraft design project.

I'm curious. How will finding such an aircraft help with a review of some other design?

@CatchAsCatchCan "If they did it so can we!" :)

The Il-76:

4 main bogies with 4 wheels each, nose gear with 4 wheels.
this one has a better view of the wheels, but it's a CGI:

4-wheel nose gear is rare, 16-wheel mains are more common: Boeing 747 (4 trucks with 4 wheels each in 2x2 config), Avro Vulcan (main trucks with 2 axles with 4 wheels each), HP Victor (same as Vulcan), Shorts Belfast (same as Vulcan).

The Antonov AN-124 has 4 nose wheels and 20 mains which is close to that configuration.
The Antonov AN-225 is also close to being configured this way. It has 4 nose wheels but has 32 mains.

(source)

That is one MASSIVE #@$§¥& airplane! Just, dang!

Not to mention a beautiful one!

C-5 has 4 front and 24 mains, but can be misconstrued as 16 from some angles.

I can only count 28 main wheels for AN-225 on that picture.

Can't find a good picture of the landing gear, but the Stratolaunch looks like it has either 24 or 48 main wheels, and 4 or 8 nose wheels.

@jamesqf there's plenty of "Stratolaunch" pictures with simple Google search. It has 24 main (6*4) and 4 front wheels.

@Alex: There are lots of Stratolaunch pictures, but none of the several dozen I looked at (the first couple of pages when searching for "stratolaunch landing gear") showed the landing gear clearly enough for me to tell whether there were single or double wheels on the gear.  (I mean that for instance each nose gear could have had 4 wheels, like the C-5.

@jamesqf I think this one is the clearest picture I could find: https://cdn.mos.cms.futurecdn.net/zAkCXEBBjgwzNTE37zLnZC.jpeg

I'd like to mention the Lockheed C-5 Galaxy, which has 4 nose wheels and 24 mains.
[source]
The large Antonovs use two two-wheel bogies for their nose gear, whereas the C-5 uses one four-wheel bogey. For the mains, the Antonovs have multiple inline two-wheel bogies, whereas the C-5 has four six-wheel bogies. As Moo commented while I was writing this, those distinctive six-wheel bogies can be deceptive from some angles and give the appearance of 16 main wheels.

A tricycle undercarriage, where the main gear comprises four tricycle undercarriages...

Lockheed know how to triforce.


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Can someone please tell me all the units for the propeller thrust equations?
This answer is excellent:-
Is there any equation to bind velocity, thrust and power?
except that it is (in practice) unusable, since all the units are missing.
I'm guessing we use meters for length (prop diameter), and watts for power.
I'm not sure what units we use for mass (grams or kilograms?) - but either way - neither of those work (grams gives answers 10x too small, and kg gives answers 1000x too large).

I have a 10,000 watt motor
My propeller is 1.25meters diameter
At full pelt, using bathroom scales on the wall, it pushes me with 35kg force

Using 0.8 for the prop efficiency, and 0.9 for the motor efficiency, and 1.225  grams/l for normal air density, the referenced formula gives 367448 grams thrust - 10 times too much.
So yeah... what are all the units supposed to be?

How far away is the prop from you?  Did you account for dispersion of that thrust, deflection off of your body curves, etc.?  Without knowing exactly how you are measuring I would guess there is a lot of wasted wind that you aren't capturing.

Thrust is force, and it should not be measured in units of mass (grams, kg).

Scales do not measure mass, so 35kg is not expressed in the correct units.

My motor is a Radne Raket Aero 120cc mounted on a nirvana colibri paramotor, so, I hung my bathrooms scales on a wall, stood in front of the scales wearing my motor on my back, and went full throttle.  I had my arm outstretched against the scales, and the push from the propeller deflected the needle by approximately 35kg.

A clever way of measuring thrust which can only be improved by mounting the scale on a pole in the open and repeat the test there, so no interference from walls will spoil the result. Still, I would expect only a small increase in thrust. The scale indicated 35 kg but actually measured 350 N. There is your missing zero.

Any good formula (that is, the one without magic numbers baked in) will accept any units as long as they are used consistently.
If you use SI, then use every figure in SI, and you'll get the result in SI.
Force is measured in Newtons, which is different to the unit of mass (kg), specifically $N =  kg⋅m/s^2$. The factor is the gravity acceleration $≈9.81\ m/s^2$ (which comes from the original definition of kg). So your measured thrust is about 343 N.
Because people are so used to treating force (weight) as mass, the 'force' units like kilogram-force (kgf) or pound-force (lbf) are commonly used, often implicitly. But this only leads to traps like you experienced and should never be used scientifically.
Density is $kg/m^3$, which is numerically equivalent to $g/l$ you used.
(In the context of propellers, one needs to be careful about the units of rotation. Strictly speaking, the SI unit of angular velocity is $rad/s$. But in practice coefficients are defined for 'n' in revolutions per second, $1/s$ (or even revolutions per minute, rpm, which is a non-standard unit). Fundamentally, radian is a dimensionless ratio, so it does not affect the resulting units (over 1/s) when used directly in formulas, and the error is not always obvious).

Brilliant!  thanks for this - the "out by 10" problem had my wondering if it's got something to do with gravity, but I wasn't sure where I'd gone wrong... Newtons, of course, was my mistake.
I notice it is a very common recurring problem in air theory that the units are always left out - the NASA web site has tons of amazing stuff, which cannot be used because of that problem. It might seem obvious to the authors what units to use, but there are SO MANY different units and things everywhere, omitting them is just condemning most of the audience to future pain when they make mistakes like me

@AnonCoward For those with a background in physics, engineering, other sciences, etc. (which I would argue is a large chunk of the audience), raw formulas with no units is the norm. It means you can use whatever units you choose or is convenient for the given problem (as Zeus demonstrated). It necessitates understanding of the formula, not just computation. A double-edged sword for those still learning, for sure.

If you're not familiar with it, it is worth reading up on dimensional analysis. It's a very simple concept that is very powerful when working with any sort of formula. More detailed Wikipedia link here

Yes - exactly - that's what makes units so powerful - you can do math on the numbers, while also doing appropriate operations on the units, which gives you a "sanity check" at the end... but only if you know the right units to begin with of course.  On that note - there IS still a units issue with that reference I gave - nothing is cancelling out the "watts" part of the result.


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Why does increasing a glider's weight using water ballast improve its speed and performance?
I was studying gliders and how they fly.  There is a point that's not clear to me: why does using water ballast improve the performance/speed of a glider?  If I understand correctly, my glider jumps from one thermal to another more quickly with ballast than without.
How can more weight improve speed?
Thanks
Ale

Yes I would say this is a dupe of the linked answer and it'll probably be closed.  It's a long read, but the short answer bottom line is ballast raises the speed at which you have your flattest glide and this helps you spend less time in sink between thermals, therefore less aggregate altitude loss and more ground covered.  You live with the higher sink rate penalty while thermaling during mid day.  Late in the day when the lift gets weak and winds diminish, your priority becomes lowering your sink rate so you can climb in weak lift to make it home, and you dump the ballast.

Welcome! Please see the linked question. If it doesn't provide a good answer for you, feel free to edit this question.


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Can anyone identify the aircraft in the background of this photo?
This photo was found in some family belongings and we're trying to date it to identify the family member who may have been in his tri-service imaged. So, can anyone identify the plane in the background please?

any info about location and/or date?


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What component of an aircraft generates its electric power?
What component of an aircraft generates the electric power to start the aircraft's engine?

For a Rotax 912 engine, I think the magneto generates the electricity, but I'm not sure; I also think about the ignition coil or electronic ignition.

Magneto generates the electric power for spark ignition, not starting.

For most small planes, it is the battery, which turns the electric starter motor. A generator driven by the engine or by a small propeller in the slipstream then charges up the battery while the plane is flying.
Some history:
a few older and small planes had a pull cord in the cockpit with which the pilot could pull-start the engine, like a lawn mower. For engines with no starter at all, the pilot or their assistant would grab the propeller and swing it around by muscle power to start the engine (be careful!).
Some very old planes had starters operated by a crank that you stuck into a hole in the engine cowling, and turned by arm power. This worked for small engines but was too hard to do for bigger engines. In that case, the crank turned a small flywheel up to great speed and then the engine was geared into the flywheel through a reduction gear drive, and the energy stored in the rotating flywheel turned the engine to start it. This was used in the P-26 "Peashooter" plane.
In later years, a small electric motor spun the flywheel up to speed instead of a hand crank. The Antonov An-2 still uses this type of starter, called an inertia starter, as does the Dromader crop duster with the radial engine.
Another method of starting an airplane engine was to explode a blank shotgun shell in a closed chamber and send the hot gas through a tube to a special cylinder geared onto the crankshaft, which thereby turned the crankshaft to start the engine. This is called a Coffman starter and it plays a key role in the old movie "The Flight Of The Phoenix". In some versions of this principle, the gases from the shotgun shell were sent right into one of the cylinders of the engine itself, forcing the piston down and turning the crankshaft.
This idea saw use in more modern times, where a jet engine could be started by burning a charge of solid rocket motor fuel called a cartridge inside a sealed chamber and using the hot gas to spin up the exhaust turbine. F-4 Phantom jet fighters could be started with cartridges like this in just a few minutes even in Alaskan winters- without the use of a start cart.

"Another method of starting an airplane engine was to explode a blank shotgun shell in a closed chamber and send the hot gas through a tube to one of the cylinders in the engine, forcing its piston violently down and thereby turning the crankshaft."
Wow!!!!

@RafaelVega, yes, it's called a coffman starter and it plays a key role in the movie "The Flight Of The Phoenix".

The battery doesn't GENERATE the electric power, it merely stores generated power for later use.  In many light planes, electricity is generated by an alternator, just as in a car.  In fact, the alternator on my Piper Cherokee was a '60s Chrysler part.

The magneto generates the spark for the ignition system. While the engine is running it drives an alternator which produces electricity to recharge the batteries and provides power to the aircrafts systems. When the engine is off, the aircrafts power is derived solely from the battery. The battery is used to power the starter motor which gets the engine going.

Critically there are two sources; the battery to turn the engine over and the magneto to generate a spark.  Although other ignition systems are commonplace in automotive engines these days, magnetos are still used in aircraft engines because they are very simple and therefore hopefully reliable.

As your question goes, "power to start the engine" may be understood as the power needed to rotate the engine until it is able to rotate itself by its own power. It may or may not be electrical in nature.
It may be the pilot's muscle power in very small engines (just like in smaller motorcycles), a starter battery for bigger engines (just like in cars) or power from a land-bound infrastructure (for big jets).
If you think about the electricity in the spark plugs of a piston engine, there is a catch - electricity for sparks is needed not only to start the engine, but for running the engine as well.
"Magneto" is just a small synchronous generator mounted directly on the engine crankshaft. It serves two purposes - providing power and timing for the electric spark. It is used in small engines (not only in aircraft) that can be started without electricity present (think motorcycle or a lawnmower). The magneto circuit is completely separate from the other possible electric equipment.
Bigger engines that are started by a battery power usually don't have a magneto. Their electricity needs are served by the starter battery (when starting) or by an attached generator (when running) - just like in most cars.
The ignition coil doesn't produce electricity. It just transforms it to the voltage needed for the sparks.
The electronic ignition is just a method to control the ignition coil. It doesn't produce electricity by itself.

A larger turbine-powered aircraft such as a Grumman G-1 Gulfstream uses an auxiliary power unit to start each of two 1800 hp Rolls Royce turbine engines.  The APU, as it is called, is a self-contained unit in the tail portion of the aircraft.  Specifically, the APU is started by battery, usually from a ground assisted cart.  The cart is essentially a truck with a self-latching-plug electrical cable connected to a male-to-female receptacle on the aircraft.  Such cart services are provided by most executive airports in the United States.  Quite typically, while the aircraft is in ground operation (pre-flight for instance) the APU powers the aircraft.  Use of the APU is discontinued when all aircraft systems are flight ready.


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What are variable stator vanes?
Most turbofans nowadays use "variable stator vanes"; what are these, what is their purpose, and how does it affect a gas turbine's performance?

Ah sorry, because both of them are in compressor airflow control so I thought they are related.

VSV and VBV are not the same. As explained in the linked question VBV are valves to discharge air into the secondary flow, VSV are vanes with a variable angle of attack, found on the first stages of the high pressure compressor (e.g. CFM56, the first 3 stators). See these slides starting p58.

Related: Does the stator blade angle change? Why?

@AnugerahRadifan they are related indeed in terms of airflow control, for example on the Trent 500s the Engine Electric Controller (EEC) controls the variable stator vanes, variable inlet guide vanes and bleed valves. The first two to control AOA and the third to prevent engine stall/surge conditions. I understand why you would include them together in the same question - as if you are trying to understand the system, you would learn about them together, rather than tackling them in isolation.

@mins on TRENTs e.g the TRENT 500, the VSVs are found in the Intermediate compressor (first stage). Three bleed values in the Intermediate Compressor and a further three bleed valves are found in the HP compressor, both controlled the EEC via the Bleed Valve Solenoid Unit.

A fan blade at the front or turbine blade at the back of a jet engine is a small aerofoil surface which is fixed to the main shaft and spins with it. By contrast, a stator vane is a similar aerofoil surface fixed to the outer casing and remains static.
Why do that? The spinning blades of a given stage cause the airflow to start spinning, and that slows the relative speed of the next stage (two things spinning together are at rest with respect to each other). The stator vanes "un-spin" the airflow so that the next stage blades meet it at full speed.
A variable vane pivots about its spanwise (radial) axis, allowing you to change its angle of attack to the oncoming airflow. Why do that? A turbojet engine at different stages in the flight will have different demands and conditions on it and will accordingly spin at different speeds. These effects change the airflow pattern through the engine. If a given stator vane is optimised for one stage of flight, it may not work so well in the modified airflow when a different stage is reached. Varying its angle can ensure that it can always be set at the optimum angle for the best performance.
So variable stator vanes ensure that a given jet engine can always operate at maximum efficiency and deliver optimal performance.
A typical way this is done is to fit gear wheels on the outer ends of the vane pivots, and place a single large toothed wheel around the whole engine so that when the large wheel turns, all the vanes in that position turn with it.

"The usual way this is done is to fit gear wheels on the outer ends of the vane pivots", CFMI uses levers and a ring actuated by bellcranks . See pages 136 et seq.

True. I have modified my answer accordingly.


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Determination of a Joukowski airfoil chord (demonstration)
I'm currently studying Aerodynamics, and one thing that I noticed is that the maximum and minimum $x$ coordinate of the airfoils (which are necessary to compute the chord) on the transformed plane (let it be $z(x,y)$) correspond to the transformed intersection points of the circumference on the original plane  (let it be $\zeta(\xi,\eta)$) with the real axis. I don't find any proof of this statement, and so I tried to do it by myself. The problem is that it got too complicated to be solved analytically (too many different powers of trignometric functions). So I'm requesting someone to try demonstrate this too. I think that there's a simpler way to do it.
There's my introduction to the problem:

Consider the original circumference defined on the complex $\zeta$ plane:

where $a$ is the circumference radius, and $b$ the intersection of the circumference with the real positive axis, $\xi$. The parameter $\beta$ is the angle between the horizontal line and the line that links $\zeta_0$ to $b$. The center of the circumference is:
$$\zeta_0=-b\varepsilon+ia\sin(\beta)=-b\varepsilon+i(1+\varepsilon)b\tan(\beta)$$
So, this circumference is defined by:
$$\zeta=-b\varepsilon+b\frac{1+\varepsilon}{\cos(\beta)}\left(e^{i\theta}+i\sin(\beta)\right),\hspace{15pt}\theta \in [0,2\pi]$$

Now I need to show that for the Joukowski transform $z=\zeta+\frac{b^2}{\zeta}$ the $x$ coordinate (real coordinate on the plane $z$) has
a maximum on $\zeta=b$ (intersection of the circunference with the positive real axis) and a minimum on $\zeta=-b(1+2\varepsilon)$, (intersection of the circunference with the negative real axis).

As I said: "The problem is that it got too complicated to be solved analytically (too many different powers of trignometric functions). So I'm requesting someone to try demonstrate this too. I think that there's a simpler way to do it."  I tried, but I didn't succeed. That's why I posted this question here.

I asked this same question on maths.SE, 1 month ago. But is still unanswered. I thought this could be due to disenterest for this type of topics, so I tried this site whch is more related to aerodynamics.

oh, I understand. good luck then.

Anyway, I am now doubting that the definition that I used for the chord is the correct one. I discovered through Mathematica simulations, that the  leftmost point of the airfoil doesn' t coincide with intersection of the airfoil with the negative real semi axis. But they are really near. Maybe an approximation to the chord is used insted of the exact value.


text:
How to justify that M_AC=0 for a symmetric airfoil?

I learnt that for a symmetric airfoil, the pitch moment around the aerodynamic center should be null:

$$M_{AC}=0$$
$\hspace{11pt}$ How could I intuitively explain this statement?

I also learnt that for an airfoil with positive curvature, $M_{AC}<0$ and  for an airfoil with negative curvature, $M_{AC}>0$. I'm new at my Flight Stability course, and I don't know how to achieve these conclusions. I do know the definition of aerodynamic center, but it isn't enough for me to make such observations.
How could I deduce these statements?

This answer should help. Let me know if it doesn't.

Per definition the moment coefficient in the Aerodynamic Centre is independent of the angle of attack.
Since the airfoil is symmetrical, the moment around the a.c., $M_{a.c.}$ for an angle of attack $\alpha $ will be of equal magnitude and opposite sign at angle of attack $-\alpha $:
$M_{a.c.}(\alpha)=-M_{a.c.}(-\alpha)$

The two statements can only hold if $M_{a.c.} = 0$

So, the second statement is true because the symmetry of the airfoil implies that the Aerodynamic Center is on symmetry axis. If the airfoil wasn't symmetric we would have two problems: locate the a.c. at horizontal axis and at the vertical axis. Without knowing one of these two coordinates it's hard to know the other. So, the answer to my second probem can't be so intuitive! Am I right?

After thinking about the question again, I should say that this answer as it is, can be considered self-contradictory. Since $M_{a.c.}$, by definition, doesn't depend on $\alpha$, it will always have the same value regardless of the signal of $\alpha$. Therefore the statement  "$M_{a.c.}$ for an angle of attack $\alpha$ will be of equal magnitude and opposite sign at angle of attack $-\alpha$" is wrong. (To be continued)

(Continuation) "Opposite sign" should be replaced by "same sign'', and no important conclusion can be taken from this. However, in reality $M_{a.c.}$ is defined for small angles, where the linearisation of the dynamic equations may be performed.  The truth is that $M_{a.c.}$ doesn't exist in exact terms. But if "$a.c.$" in "$M_{a.c.}$"' were substituted by some point in the chord of the airfoil near the "$a.c.$" point that would be obtained in a linear world, then the answer can be considered right.


text:
Have jet engines ever "Sheared" off?
I have read that jet engines, at least the under-wing ones, are attached to the airplane by shear nuts and bolts so that in case of unstable forces in or on the engine, the engine would fall off to prevent further damage to the airframe. Have there been any incidents and or accidents where the engine has fallen off the plane in accordance to this design feature?

American Airlines flight 191 , and Investigation video had that happen. There are quite a few examples of engine separation,  both fatal and nonfatal if you search on Google.

When I was flying there was a magazine with a feature about close calls - I can't remember if it was Flying or AOPA Pilot...but there was a story about a crew in Alaska flying at night that had their engine catastrophically fail and, as the pilot told ATC (who thought this was an engine failure), "the engine departed the airframe." It may have been a prop driven aircraft, but the article was fascinating nonetheless...

Not only in reality but even in movies: see Donnie Darko.

Where can I read about the requirement for shedding engines? Isn't turning it off much safer than ejecting significant debris?

@Gusdor Its not a requirement in that its mandated by a regulatory body, its a design feature.

@Gusdor The closest think to a "requirement" in the US that I can find is 14 CFR 25.571 Which outlines the requirement for damage tolerance, one part of which is the engine mounting structure.

Too lazy to search now but I witnessed it as a kid. Short after takeoff from Recife - Brazil a boeing (cannot remeber details by I guess it was a 727) lost one engine and it crashed a few yards from a house in a rural area. Two and half hours later the airliner itself landed safely on São Paulo. To add bizarre a report asked the pilot about the incident and he said the crew not even noticed losting an engine before landing.

@jean: Was it this one? http://aviation-safety.net/database/record.php?id=19870115-2

@FredLarson Maybe, as I said cannot remember the details but ill search for similar matches shame there's no youtube at the time =) wish I can find the original news on tv

@FredLarson Not found any other similar reference but also don't found another two incidents (Aerolineas Argentinas 707 belly landing and a F14 ditched to make a hard landing with falty landing gear). Living next to a busy airport makes you to witness lots of incidents. Maybe later I can search local newspapers archives =)

@jean Hmm... The pilot not noticing an engine completely missing seems rather hard to believe. It seems like the engine instruments all reporting zero and the need to apply large rudder inputs in order to fly straight would be pretty noticeable.

@reirab: I don't think the pilot would have needed "large rudder inputs". And as for "the engine instruments all reporting zero", wouldn't that be expected from a flamed-out engine?

@RonBeyer That link is not working for me.

@MartinArgerami I took his comment as meaning they hadn't noticed one of the engines wasn't working (which seems really hard to believe.) At any rate, I'd assume that a jet would report things like oil level and various temperatures, which you wouldn't expect to suddenly all report no information at all in the case of a flame-out. It would also take a bit for the engine to spin down, so N1/N2 shouldn't immediately drop to zero. Also, why do you say large rudder input wouldn't be required to counter the yawing moment from asymmetric thrust?

@kasperd It works for me, but it's a PDF. Here's a non-PDF source.

@reirab: because in a 727 the engines are really close to the fuselage. As for the behaviour of the engine sensors during an engine failure, I cannot comment on that, but I can easily image that there are failures where everything goes to zero immediately.

When an engine seperates more or less cleanly (without damaging too much of the wing structure and control surfaces) I can imagine it means less trouble because there's less drag than when an inoperative engine stays in place. Less assymetry, easier handling of what's left of the aircraft. A good thing - from the perspective of crew and passengers of course, not for folks and things on the ground.

@reirab To settle it Fred Larson finding is probably the right one. Cannot find old (30 years) news archives about that incident and unfortunately I don't have access to old tv news.

Engine shearoff, though rare, has happened in a number of cases.

Boeing 747F experienced a number of engine falloffs:

On Dec 1991, China Airlines flight 358, a 747-200F, lost an engine near Taiwan and crashed.
On October 1992, a El Al 747-200F crashed after takeoff due to engine separation, at Amsterdam, the Netherlands.
Another 747F from Evergreen Airlines lost an engine over Anchorage, Alaska soon after.
On October 2004, a Boeing 747-132SF of Kalitta Air lost an engine while climbing and landed without further incident.

Engine separation has been reported on 737s too:

In November 2007, Flight CE723, a Nationwide 737-200 lost an engine during takeoff; the aircraft was landed without further incident.
On December 1987, USAIR FLT 224, B737, lost an engine during climb; the aircraft was landed successfully.

There are cases where engine separation has occurred in B707, B727, and DC-10. aviation-safety.net has a list of engine separation incidents.

CE723: “I heard this huge bang, and he said ‘that’s our engine that’s just fallen off’" Thats why you train for an engine failure at V1 :)

The El Al flight tried to return to Schiphol and didn't make it because control structures failed... and it ended up inside an apartment building. Being a cargo plane with a crew of just 4, most of the fatalities (39 of 43) were on the ground.

Strange that this "safety feature" seems to have resulted in as many aircraft losses as it saved. That's a pretty bad success rate.

@James allowing the engines to shear off implies that if you don't detach just the engine, it will destroy the entire wing/airframe due to the extreme forces acting on it. Losing an entire wing is almost entirely a death sentence. Losing an engine and facing 50/50 odds seems like a much better option.

@James: It's mostly to protect the wing's fuel tanks during a crash or hard landing (so that the engine comes off cleanly at the pylon, rather than potentially ripping a hole in the wing); the fact that it occasionally allows engines to come off in flight is a side effect (a bug, not a feature).

ELAL flight 1862 crashed in Amsterdam on the 4th of October 1992 as a result of 2 engines shearing off. The number 3 engine sheared off shortly after take-off due to metal fatigue related failure of the fuse pins. After separation, the number 3 engine hit the number 4 engine, shearing it off as well. Combined with damage to the leading edge of the right wing the aircraft became difficult to control at low speeds and subsequently crashed on the attempt to return to Schiphol Airport.
Official accident investigation report (PDF)

The Boeing 727 acquired a reputation for shedding engines. The process was ice buildup on the right side due to leaking lavatory plumbing, the ice falling off and into the No. 3 engine where it caused damage to the fan blades. The resulting imbalance lead to vibrations, and the engine was designed to shear off in that case.
This New York Times article describes one case where a 727 lost its engine over Florida. The article continues:

Cases of engines falling from airplanes are rare. In 1974 a National
  Airlines 727 lost an engine near Sierra Blanca, Tex. In 1985 an
  American Airlines 727 flying from Dallas to San Diego lost an engine
  near Deming, N.M.

I would expect this happened also to some Russian airplanes, but here the reporting is less up-front, so I know of none.

Leaky toilet -> Engine falling off of wing.
That escalated quickly...

In addition to the cases where engines fell off in flight, there have also been cases where the engines sheared off due to impact forces during a crash, which is the primary reason for attaching to the wing with shear pins. This helps to prevent excessive loads from breaking the wing structure, which would cause a fuel leak from the tanks in the wings.
Some examples:
July 2013, an Asiana 777-200ER crashed short of the runway. Both engines separated.
April 2013, a Lion Air 737-800 crashed short of the runway. The right hand engine sheared off.
February 2009, a Turkish Airlines 737-800 crashed, both engines separated.
December 2008, a Continental 737-500 departed the runway on takeoff. One engine separated.

And as a negative example, in https://en.wikipedia.org/wiki/Air_Canada_Flight_621, failure of the engine to separate cleanly led to a fire in the wing and a fatal crash...

I don't get it.  You're saying that it's a design goal that engines can shear off during a crash?  I don't see why that is even desirable.

@NateEldredge possibly removing as much mass (therefore energy) as possible helps? And the fewer massive, possibly still-spinning turbines near passengers the better.

@NateEldredge -- having the engine mount fail in a controlled way protects important wing structures from being damaged by the engine tearing off in a crash.

@NateEldredge There are fuel tanks in the wings. If the engine strikes something during the crash, or is dragging along the ground, it might break the wing and spill fuel. Designing the engine to shear off avoids this problem.


text:
Is there a new "best practices" technique for demonstrating stall recovery?
This is a question that is probably best suited for active flight instructors. I was doing an FBO checkout and the CFI asked me to demonstrate power on and power off stall recoveries, but well before the stall he asked me to recover. His explanation is that it is the new best practices for teaching and demonstrating stalls to recover "before" the aircraft actually stalls. In essence, the recovery demonstrations consisted of setting up the plane for landing and take-off config, and slowing it down till the stall warning came on, then almost immediately "recovering." It seems too easy. However, his reasoning made sense, but I have never heard it before:

It prevents new pilots from getting comfortable flying with the stall warning buzzing.
It teaches pilots to avoid the stall in the first place. (Prevention)
Its safer.

While his points are all factually correct, I think this defeats the purpose of knowing how to recover from a stall.
Any current CFIs out there that can explain if this is a new "best practice" for demonstrating stall recovery?

Closely related, although it seems there may be some more recent guidance

It looks like its related to this safety alert for operators that was issued back in may. You can find the full text from the FAA here but the pertinent chunk for the private pilot is...

(1) Private Pilot – Airplane ACS, Power-Off & Power-On Stalls. With
  the exception of performing a thoroughly briefed full stall maneuver,
  a pilot should always perform the stall recovery procedure when a
  stall warning occurs

Going into a fully developed power-on stall in my short-time in flight school was one of the highlights! I distinctly remember the feeling of hanging in the harness pointed basically straight at the ground (a few thousand feet up of course). Or maybe I remember it with rose colored glasses...

So the kid was right. Great answer Dave!

@Devil07 looks to be that way, although my flight school/FBO (just did my 90 day rental checkout) is still doing full break stall training and testing. Frankly I think its important that student pilots see the full break but thats just me, the FAA clearly has their reasons and their data.


text:
Is it possible to revive a pilot who has had a medical condition in a seated position in a small aircraft?
I recently learned of a business executive who sold his company jet (single pilot operation) after his corporate pilot (in his 50ies) fell to the ground during the pre-flight inspection of the aircraft and died. He and his family were about to go on a trip. Very lucky guy. Obviously, he could have just paid for a qualified 2nd pilot to always fly in the right seat, but thats not the point of this question.
It got me to thinking. Is it possible to revive a pilot who has stopped breathing and/or who's heart has stopped beating in a small aircraft in a seated position? Assuming of course aircraft is in cruise with plenty of altitude.

I don't think this question is answerable. It depends entirely on what was wrong with the pilot. Fainted from hyperventilation, yeah. Massive coronary, no.

A qualified answer will be acceptable (i.e. CPR for respiratory arrest is possible in seated position, etc.).

This depends on the medical condition of the pilot and (sorry to say) has nothing to do with aviation. Perhaps Health.SE is better?

CPR won't work in a seated position. You can't produce enough pressure to move blood to the brain. You have to lie the victim down on a hard surface to even have a chance. Even at that, some studies have shown that only about 2% of people who receive CPR outside of a hospital end up surviving long enough to be released from the hospital

It is an aeromedical question, but feel free to down vote it if you don't approve.

It's theoretically possible but highly unlikely. In most cases where the pilot died, the other individual was too busy flying the plane to attempt to revive them.

I'm going to preface this by saying I'm not a doctor, however I am trained in first aid and CPR.
If you are asking whether it would be possible to revive someone who is in respiratory or cardiac arrest to the point where they could fly again then the answer is absolutely not.
The movies make it seem like you can give CPR and 2 minutes later the person is running around shooting guns, however that's very far from the truth. CPR and rescue breathing are techniques which can, if done properly, keep someone's blood moving enough that radical emergency medicine may save their life. CPR cracks and/or breaks ribs and can cause internal injuries even when it's done right, rescue breathing doesn't generally cause injury, however anyone who has needed either will require days, weeks or even months of intensive care and recovery.
As to whether it would be possible to administer CPR or rescue breathing to someone in a light airplane then I would say yes, with extreme difficulty and a low chance of success. CPR requires a lot of force, you use the full weight of your upper body concentrated on one single point on the person's chest, unless you could lay the person flat on a hard surface you'd probably be wasting your time, so if the seats reclined a person in the back seat may be able to lean over and do it, although that's extremely chancy of working due to the angle and the cushioning of the seat. Rescue breathing is possible with a seated person, provided you can open the airway, getting the angle on that would be very tricky in the constrained area of a small cockpit. Again, if the seat could recline a person in the back would have a better chance of doing this.
Most importantly to remember is that if the sole pilot dies or becomes incapacitated someone else has to fly the airplane. Struggling to get into position to perform CPR or mouth to mouth on a seated person would be a great way to snag the stick and send the airplane out of control. If there's a back seater by all means let them give it a shot, but your best bet at survival is getting control of the airplane and calling ATC for someone to talk you through things, it's happened before and has a remarkably good survival rate.

Good analysis. I have CPR certification also, and I was trained that they must be on a hard surface for compressions to work. I agree that position and cushion of seat probably makes CPR not effective. Although, like you said, rescue breathing might work if heart is still good. According to Google, some of the conditions giving rise to rescue breathing include, among other things, carbon monoxide poisoning, severe asthma attack, and choking. These are things that could affect a pilot, but it sounds like you're saying anything that stops heart could not be addressed.

That about sums it up @Devil07

"...calling ATC for someone to talk you through things, it's happened before and has a remarkably good survival rate." For the passengers that is, the pilot who suffers cardiac arrest in the cockpit has a remarkably low survival rate :)

Cardiac arrest tends to not end well @RonBeyer.

Was very tempted to downvote for the missed opportunity to open this with, "dammit man, I'm a CPR-trained pilot, not a doctor!" ;)


text:
Is it appropriate (legal) to request practice instrument approaches in IMC?
I have always requested practice instrument approaches while maintaining VFR (with a safety pilot or instructor). But I was wondering if there are any issues (legal or otherwise) with requesting multiple approaches in actual weather at the end of a short IFR flight. Assuming, there are no safety or controller workload issues. In other words, assuming weather is well above minimums and controlling facility is not busy (during off peak hours).
I realize I could just make multiple missed approaches, and get the extra approaches that way, but that is not what I'm asking.

I assume you either have your IR or would be flying with someone who does?

Yes. Instrument rated and current. I just never thought about this or have ever heard of anyone doing it, but can't think of a reason why it wouldn't be permitted assuming safety and workload issues permit as mentioned.

I suppose you could make a request to the controller. They could always just deny it

Please do request it if you are planning to perform multiple approaches. "I could just make multiple missed approaches" - this would be really annoying for the controllers. We really like to be able to plan ahead, so please tell us what your plan is.

@J.Hougaard that is precisely what I was thinking. Just like pilots, controllers prefer to plan ahead.

To second @expeditedescent, if you're in a turbojet aircraft an unplanned go-around near to the threshold requires paperwork on our end. If you want several approaches, just let us know and we'll try to get you what you want.

Approaches in actual IMC aren't really "practice": you're an IFR aircraft doing IFR stuff, which just happens to be multiple instrument approaches.  It isn't anything to be ashamed of - everyone in the system learns by doing at some point.  The controller you're talking to may be a trainee, too, and learning something at the same time you are.  It's nice to be considerate and not show up in the middle of an inbound rush to a major airport (or even an airport close to a major airport, since controllers are responsible for a wide area and you'll be part of their workload regardless...), but they get paid to provide a service and you're entirely within your rights to request multiple approaches.  Military aircraft do that all the time - for example, a B52 coming back from a 7-hour training flight may spend another hour or two doing approaches with a civil approach control before landing.  Just tell ATC what you want to do, and pay attention in case they issue alternate missed approach instructions.

That is a good point. Its not a "practice" approach, its a request for multiple IFR approaches. The fact that I'm doing it for practice is only from my perspective. Point well taken.

There is no problem as long as you tell ATC your intentions.
If you request an “approach”, they expect you to make a full stop. If you’re IFR, they will protect the miss just in case, but they’re not expecting you to actually use it. (Note: they don’t protect it if you’re VFR.)
However, if you request a “practice approach”, they expect you to miss. It’s best to tell them your intentions at the same time, e.g. “practice ILS 18, next is RNAV 18”, so they can give you alternate miss instructions that will set you up for the next approach. If you don’t tell them what’s next, they have to ask anyway, so save yourself (and them) the extra radio chatter. When you’re done practicing, just tell them that on your last approach, e.g. “VOR A circle 18, full stop”.

They absolutely will let you fly practice instrument approaches in IMC.  You will have to be an instrument rated pilot with an IFR equipped aircraft and file a local IFR flight plan with your intended route of flight and approaches with terminations.
Along with full motion simulator time, this is a good way to maintain IFR currency, especially if you live in an area with frequent and harmless IFR conditions e.g. low stratus clouds free of icing, embedded thunderstorms, etc.  in addition flying multiple approaches and holds, one right after the other, is more work intensive than a real IFR CC flight in IMC.

If you have an instrument rating or are under training from an IR rated instructor then there's no problem asking for as many instrument approaches as you want. It's good training to fly in real IR conditions rather than simulated. There's nothing like real cloud to concentrate the mind.
It would be very much against the rules to fly in instrument conditions without an IR though.


text:
How can a CFI renew their certificate with the FAA during the government shutdown?
My CFI certificate expires at the end of the month. I have completed the required Flight Instructor Renewal Course ("FIRC"), but when I called my local FSDO to make an appointment to submit my renewal application for a new CFI certificate I discovered that the FAA is shut down. If the shutdown continues through my CFI expiration date, can I renew my certificate with the FAA after the expiration date?

What is the name of the policing agency that prevents you from operating without the certificate?

@A.I.Breveleri I'm not sure if that was a serious question, but this may be relevant. It's also worth considering that your insurance company will refuse coverage if they have evidence that you weren't operating in accordance with the regulations. Giving instruction without a valid CFI certificate world certainly be in that category. That could turn a minor incident into a cripplingly expensive one.

My question was intended as a potential trigger for what I hoped would be a serious discussion - Thank you for your informative remark.

Short answer is no, but there is a way to renew your CFI while Federal Government is shutdown. However, it's not through the FAA because so long as the FAA is shut down it cannot process applications.
(After a few hours of freaking out at the thought of having to redo my CFII check ride, I discovered there is a solution to this.
While the FAA is not processing applications during the government shutdown, the FAA has authorized certain non-federal employees to accept and process airmen applications for CFI renewal AND to issue temporary flight instructor certificates. These are called Airman Certification Representatives (ACR), and are defined as:

Airman Certification Representative (ACR). An ACR is an individual representing a specific FIRC provider who is authorized under Title 49
  of the United States Code (49 U.S.C.) § 44702(d) to accept airmen
  applications for renewal of a valid flight instructor certificate from
  successful graduates of that provider’s FIRC and to issue temporary
  flight instructor certificates

Source: Advisory Circular 61-83J.

You may also find a DPE who is authorized to renew the certificate as well.


text:
How much effect does the nose design of the plane have?
Boeing 747 nose

Which looks like a goose head

which, for me, much uglier than 737

which has a head similar to
which is of course, much better looking from my point of view. Why are there different designs of the nose for planes by the same manufacturers? What effects do the different designs have?

Related

Do you mean the nose design? "Head" isn't a common term for a part of the plane.

Your pictures don't really show nose design, but rather fuselage cross-section. That is almost always circular as that provides highest structural strength, 747 being exception moving the cockpit up to allow full-height cargo door for the main deck.

There is other difference that your images do not show; Boeing aircraft have the nose more pointy and slightly squashed from the sides (especially the 737) while Airbus aircraft have it more rounded.

Which is of course, much better looking is quite subjective. I'll bet that gooses' head is pretty darn sexy looking to other geese.

@FreeMan As I have added "from my point of view", but that's not the most important part of the question, rather the effect of it is.

I thought the question meant "effect on the pilot's field of vision" - since, when landing, a 747's cabin is quite some distance above the ground when the main gear contacts. Agree we need more precision in the question.

I actually didn't mean it about the looks, but about the efficiency and performances of the plane and the pilots both.

closely related. I recommend you read answers and comments and then clarify the difference you're asking for (I guess you're asking about cross section due to the upper deck)

The nose design can have an effect on the aircraft but that depends on the type of aircraft.
For small single engine GA aircraft the nose area behind the propellor is critical. Generally known as the cowling over the years it has has many different designs. The varying designs have had drastic effects on engine cooling and drag depending on the plane. Early Mooney's suffered from a less than idea cowling and mods gained them a lot of speed.
For jets like the ones you have pictured there are few things to  consider in nose cone design. First off (although this has no aerodynamic effect). There is equipment housed behind the nose cone. Often radar and other things so the nose generally needs to able to accommodate that. Drag is a consideration when designing the nose as well, here is a nice chart regarding the drag on various designs.

There is a pretty lengthy paper on missile design that covers nose cones a bit as well.
For supersonic planes there are other considerations as well such as the shock wave produced and other things that are out of my body of knowledge but Im sure someone here can weigh in. One very interesting nose is that of the Concorde. On one hand it had its awesome moveable system which allowed for greater visibility on takeoff and landing and a more streamline (and heat treated glass) for super sonic flight. But perhaps more interestingly the plane had a serious nose heat issue. They solved this by pumping fuel through the nose as a form of coolant.

Good question! The raised cockpit and the hump behind it make the 747 look like the goose. Raising the cockpit allows the hinged nose to come up to load freight. The raised area behind the cockpit was initially a lounge, and was then used for seating or cargo.
How the 747 Got Its Hump, Smithsonian Air and Space
Boeing 747-8F:
Photo by Olivier Cleynen

And how does it answer the effects of the noise on plane design in general?

Not a huge effect. The raised cockpit isn't very important. The additional cross-sectional area is partially offset by the lift generated by the curved top of the fuselage.